An image sensor is a device that may receive light at a photodiode or phototransistor and converts the light into an electrical, digital image signal using an analog to digital converter (ADC). Two different types of image sensors are Complementary Metal Oxide Semiconductor (CMOS) devices and Charge Coupled Devices (CCDs). CMOS devices may be used in low power, small cameras. In CMOS devices, micro lenses (which may be exposed to the air) may focus light on a photodiode. The photodiode may convert the focused light into electrical signals. A plurality of transistors may cause the converted electrical signal to be displayed as an image.
A CMOS device may include a photodiode, at least one transistor, at least one metal wiring layer, and/or at least one interlayer dielectric layer. A metal wiring layer may electrically connect circuit devices in a CMOS device. An interlayer dielectric may insulate circuit devices and metal wiring layers. A CMOS device may include a color filter array including a plurality of color filters. Each color filter may allow transmission of light having a predefined wavelength range, which may correspond to one of the three primary colors (i.e. red, blue, and green). Color filters may be formed over and/or on an upper interlayer dielectric layer. A CMOS device may include a plurality of microlenses corresponding to the plurality of filters formed over and/or on the color filter array. A CMOS device may include a flattening layer that compensates for the difference in height between the color filters. A flattening layer may be between the plurality of microlenses and the color filter array.
Microlenses may be formed to maximize the effect of the color filter array. To form the microlenses, predetermined patterns may be formed using a relatively highly transparent photoresist. Photoresist patterns may be formed into microlenses through a thermal process.
Example FIGS. 1A to 1C illustrate a method of forming a microlens. As illustrated in example FIG. 1A, a red filter 10a, a green filter 10b, and a blue filter 10c may be formed over an upper interlayer dielectric layer of an image device. The upper interlayer dielectric layer and at least one metal wiring layer may be formed over a semiconductor substrate. A photodiode and a plurality of transistors may be formed on and/or over the semiconductor substrate.
A flattening layer 20 may be formed on and/or over color filter array 10 to compensate for differences in height between the filters 10. Photoresist film 30 may be formed on and/or over flattening layer 20. Photoresist film 30 may be exposed to light through mask 50 having a predetermined pattern to shape microlenses to have hemispherical shapes. Defocus exposure may be performed to allow unexposed portions 30b of the photoresist film 30 to have inclined sides. Unexposed portions 30b may be removed through development, thereby forming photoresist patterns 30b having a trapezoidal shape. As illustrated in FIG. 1C, photoresist patterns 30b are subjected to reflow and/or hardening processes to form microlenses 40 having substantially a hemispherical shape.
The features or characteristics of CMOS image sensors may be affected by combinations of different factors, such as focusing distance of microlenses, size and degree of distribution of color filters, thickness of interlayer dielectric layers, and/or pitch of photodiodes. It may be difficult to standardize microlens focusing distances due to variations in a manufacturing process, thus limiting reproducibility of microlenses. For example, it may be difficult to standardize a reflow process. For example, photoresist patterns 30b of FIG. 1B should be formed far enough away from each other to prevent bridges from being formed. Likewise, microlenses 40 should have enough separation from each other to avoid crosstalk.